Dual Xeon vs High-End GPU for Cycles rendering?
I use Blender at work. This might help you out.

My work computer used to be a Mac Pro with a hyper-threaded Xeon processor. It was a little older, like from 2010, but it could run 16 simultaneous threads. That translated to 16 little squares rendering at the same time in Cycles.

A year and a half ago I asked my boss if I could build a PC instead of getting a new Mac Pro. They approved and gave me a budget of \$3500, roughly the amount a new Mac Pro would have cost. Because I was choosing my components I bought two GTX 980s (they were about \$550 a piece, so \$1100 total).

When I compared the Cycles rendering speed on the two computers, the PC I built rendered the same scene SIXTY TIMES FASTER than the Mac Pro with the Xeon processor. That meant projects that used to take multiple week to render would now be done over night.

I used Render.st once. It was a very smooth experience, and didn't cost me anything because they had given me a \$25 credit to get started. It wasn't blazing fast, but it did render for much less money than the cost of building and maintaining my computer. And like Pyroglyph mentioned, all the frames began and ended around the same time.

If you are rendering things every now and then, for fun, I suggest trying a render service like Render.st. It's very easy to set up.